U.S. patent application number 12/601657 was filed with the patent office on 2011-02-10 for magnetic energy recovery switech having protective circuit.
This patent application is currently assigned to MERSTECH INC.. Invention is credited to Ryuichi Shimada.
Application Number | 20110032652 12/601657 |
Document ID | / |
Family ID | 40985172 |
Filed Date | 2011-02-10 |
United States Patent
Application |
20110032652 |
Kind Code |
A1 |
Shimada; Ryuichi |
February 10, 2011 |
MAGNETIC ENERGY RECOVERY SWITECH HAVING PROTECTIVE CIRCUIT
Abstract
In order to protect Magnetic Energy Recovery Switch (MERS)
against an overvoltage and an overcurrent a voltage detection unit
is provided for detecting the capacitor voltage of the MERS and
control means is provided to control so as to turn ON the switch of
the discharge circuit connected in parallel with the capacitor to
make the capacitor discharge the electric charge thereof when the
output of the voltage detection unit exceeds a predetermined value.
Moreover, a current detection unit is interposed between the AC
power supply and the load for detecting the current flowing to the
load, and the current limiting is carried out by making the duty
ratio of the ON/OFF of the pulse of the gate control signals of the
MERS switches smaller than 0.5, when the output of the current
detection unit exceeds a predetermined value.
Inventors: |
Shimada; Ryuichi; (Tokyo,
JP) |
Correspondence
Address: |
INTERNATIONAL KNOWLEDGE ASSET OFFICE
19200 Von Karman Ave. Suite 400
IRVINE
CA
92612
US
|
Assignee: |
MERSTECH INC.
Tokyo
JP
|
Family ID: |
40985172 |
Appl. No.: |
12/601657 |
Filed: |
February 20, 2008 |
PCT Filed: |
February 20, 2008 |
PCT NO: |
PCT/JP2008/053340 |
371 Date: |
November 24, 2009 |
Current U.S.
Class: |
361/91.2 |
Current CPC
Class: |
H02M 5/293 20130101;
H02J 3/1814 20130101; Y02E 40/18 20130101; H02M 7/217 20130101;
H02M 1/32 20130101; H02M 2003/1555 20130101; Y02E 40/10
20130101 |
Class at
Publication: |
361/91.2 |
International
Class: |
H02H 3/20 20060101
H02H003/20 |
Claims
1. A magnetic energy recovery switch having a protective circuit
and interposed between an AC power supply and a load to protect,
against an overvoltage or an overcurrent, a magnetic energy
recovery switch with a variable reactance function for turning
ON/OFF of the AC current and changing a phase thereof, the magnetic
energy recovery switch comprising: a bridge circuit constructed
from four reverse-conductive type semiconductor switches; a
capacitor connected between DC terminals of the bridge circuit,
recovering and storing magnetic energy at cut off of a current; and
control means; wherein the protective circuit comprises: a voltage
detection unit connected in parallel with the capacitor and
detecting a voltage of the capacitor; a discharge circuit connected
in parallel with the capacitor and having a discharge resistor and
a discharge switch connected in series; and a current detection
unit interposed between the AC power supply and the load and
detecting a current flowing to the load; wherein the control means:
controls a phase of a gate control signal such that each of the
four reverse-conductive type semiconductor switches is controlled
so as to be paired with diagonally opposed one, such that the
diagonally opposed pair of reverse-conductive type semiconductor
switches are simultaneously turned ON/OFF in synchronization with
the voltage of the AC power supply; controls the gate of the
discharge switch so as to short-circuit the discharge switch to
discharge the electric charge of the capacitor through the
discharge resistor when an output of the voltage detection unit
exceeds a predetermined value; and controls to limit a current by
making the duty ratio of the ON/OFF of the pulse of the control
signals smaller than 0.5 when the output of the current detection
unit exceeds a predetermined value.
2. The magnetic energy recovery switch having a protective circuit
according to claim 1, wherein the control means controls gates of
the four reverse conductive type semiconductor switches so as to
turn OFF all the four reverse conductive type semiconductor
switches to cut off the current when a cumulative length of time
during which the discharge switch is short-circuited exceeds a
predetermined length.
3. The magnetic energy recovery switch having a protective circuit
according to claim 1, wherein the control means controls the gates
so as to turn ON all the four reverse-conductive type semiconductor
switches at a timing at which the voltage of the capacitor becomes
zero, so as to allow a bilateral current conduction, when a
cumulative length of time during which the discharge switch is
short-circuited exceeds a predetermined length.
4. The magnetic energy recovery switch having a protective circuit
according to claim 1, wherein the control means controls the gates
so as to turn OFF all the four reverse-conductive type
semiconductor switches when the output of the current detection
unit exceeds a predetermined value.
5. The magnetic energy recovery switch having a protective circuit
according to claim 1, wherein the control means controls the gates
so as to turn OFF only one of the two pairs of opposed
reverse-conductive type semiconductor switches that is turned ON to
cut off the current, when the output of the current detection unit
exceeds a predetermined value.
6. The magnetic energy recovery switch having a protective circuit
according to claim 1, wherein the control means comprises a
communication unit to perform communication with an external
device.
7. (canceled)
8. The magnetic energy recovery switch having a protective circuit
according to claim 2, wherein the control means comprise a
communication unit to perform communication with an external
device.
9. The magnetic energy recovery switch having a protective circuit
according to claim 3, wherein the control means comprise a
communication unit to perform communication with an external
device.
10. The magnetic energy recovery switch having a protective circuit
according to claim 4, wherein the control means comprise a
communication unit to perform communication with an external
device.
11. The magnetic energy recovery switch having a protective circuit
according to claim 5, wherein the control means comprise a
communication unit to perform communication with an external
device.
12. An AC power supply unit for supplying an AC current to a load
and recovering a magnetic energy at a time when a current is cut
off and utilizing such recovered energy as supply current to the
load, wherein the AC power supply unit has a magnetic energy
recovery switch having a protective circuit according to claim 1
connected in series between the AC power supply for generating the
AC current and the load.
13. An AC power supply unit for supplying an AC current to a load
and recovering a magnetic energy at a time when a current is cut
off and utilizing such recovered energy as supply current to the
load, wherein the AC power supply unit has a magnetic energy
recovery switch having a protective circuit according to claim 2
connected in series between the AC power supply for generating the
AC current and the load.
14. An AC power supply unit for supplying an AC current to a load
and recovering a magnetic energy at a time when a current is cut
off and utilizing such recovered energy as supply current to the
load, wherein the AC power supply unit has a magnetic energy
recovery switch having a protective circuit according to claim 3
connected in series between the AC power supply for generating the
AC current and the load.
15. An AC power supply unit for supplying an AC current to a load
and recovering a magnetic energy at a time when a current is cut
off and utilizing such recovered energy as supply current to the
load, wherein the AC power supply unit has a magnetic energy
recovery switch having a protective circuit according to claim 4
connected in series between the AC power supply for generating the
AC current and the load.
16. An AC power supply unit for supplying an AC current to a load
and recovering a magnetic energy at a time when a current is cut
off and utilizing such recovered energy as supply current to the
load, wherein the AC power supply unit has a magnetic energy
recovery switch having a protective circuit according to claim 5
connected in series between the AC power supply for generating the
AC current and the load.
17. An AC power supply unit for supplying an AC current to a load
and recovering a magnetic energy at a time when a current is cut
off and utilizing such recovered energy as supply current to the
load, wherein the AC power supply unit has a magnetic energy
recovery switch having a protective circuit according to claim 6
connected in series between the AC power supply for generating the
AC current and the load.
18. An AC power supply unit for supplying an AC current to a load
and recovering a magnetic energy at a time when a current is cut
off and utilizing such recovered energy as supply current to the
load, wherein the AC power supply unit has a magnetic energy
recovery switch having a protective circuit according to claim 7
connected in series between the AC power supply for generating the
AC current and the load.
19. An AC power supply unit for supplying an AC current to a load
and recovering a magnetic energy at a time when a current is cut
off and utilizing such recovered energy as supply current to the
load, wherein the AC power supply unit has a magnetic energy
recovery switch having a protective circuit according to claim 8
connected in series between the AC power supply for generating the
AC current and the load.
20. An AC power supply unit for supplying an AC current to a load
and recovering a magnetic energy at a time when a current is cut
off and utilizing such recovered energy as supply current to the
load, wherein the AC power supply unit has a magnetic energy
recovery switch having a protective circuit according to claim 9
connected in series between the AC power supply for generating the
AC current and the load.
21. An AC power supply unit for supplying an AC current to a load
and recovering a magnetic energy at a time when a current is cut
off and utilizing such recovered energy as supply current to the
load, wherein the AC power supply unit has a magnetic energy
recovery switch having a protective circuit according to claim 10
connected in series between the AC power supply for generating the
AC current and the load.
Description
TECHNICAL FIELD
[0001] The present invention relates to a magnetic energy recovery
switch connected between an AC power supply and a load and having a
protective circuit for protection of reverse-conductive type
semiconductor switches constituting the magnetic energy recovery
switch against overvoltage and for protection of semiconductor
switches and a load against overcurrent.
BACKGROUND ART
[0002] Nowadays electric power energy systems are critically
important social infrastructure which cannot be stopped even for a
moment. However, in an abnormality or a trouble of a load that
causes an overcurrent, a measure taken thereagainst is a high-speed
breaking of the load, as is exercised by a fuse or a high-speed
mechanical switch. Nevertheless, there has been a demand for a
highly functional switch, so called a controller or a current
limiter, which is capable of limiting only the overcurrent and
allowing a continued operation without the complete stop of the
load, as well as a system recovery to a full operation after the
return to its normality.
[0003] Electric power system must be designed to withstand a
short-time overcurrent that can be caused due to such as a rush
current of an incandescent lamp being lit, a start-up rush of an
induction motor or an initial excitation inrush of a transformer.
It is important to distribute yield capacity of each machine
appropriately. A semiconductor-type inverter power supply in recent
years, such as a fuel cell inverter, for example, cannot withstand,
in many cases, a peak current which is almost ten times the
excitation inrush current of a transformer. Inverter power
supplies, therefore, have various soft-start functions, which work
if there is one load for one inverter power supply but may not work
for late-started ones of a plurality of loads connected to one
inverter power supply.
[0004] Electric power systems are designed in consideration of
protective coordination, the current and the duration thereof to
withstand an accidental, short-time overcurrent. However, such
systems merely perform a protective coordination aimed at a
prevention of the influence over the upstream by selectively
breaking the accident current by a switch. It is a recent social
demand to achieve a continuous operation as far as possible without
power breaking if the accident has taken place in the downstream of
a system.
[0005] As for a current limiter which limits an accident current
with series elements, an application based on a transient
phenomenon between superconductive mode and normal conductive mode
is developed. This is for size and cost reduction of the breaker,
whose required capacity is extremely large when the accident
current is excessively large: such size and cost reduction can be
attained if only the accident current is reduced into the half or
so by the current limiter.
[0006] In the case of a three-phase transformer, an exciting rush
current due to iron core saturation is expected. Therefore, it is
necessary that such transformers be constructed to possess an
overcurrent withstand capacity against electromagnetic force of the
electric wire.
DISCLOSURE OF INVENTION
Problem to be Solved by the Invention
[0007] Magnetic energy recovery switch is a switch which can
control electric power between an AC power supply and a load. The
magnetic energy recovery switch has a structure of switches which
are turned ON/OFF independently by four gate signals, and can store
in a capacitor and regenerate magnetic energy of a load. It is
already patented and disclosed (see Japanese Patent No. 3634982)
that various electric power controls are possible by making a
voltage automatically generated in a capacitor to generate a
reactance voltage. It is a characteristic of this switch that the
lead of phase of the current is achieved by control of the opposed
pair of four reverse-conductive type semiconductor switches
connected in a bridge structure so that these opposed pairs turn ON
and OFF alternately by being supplied with their respective gate on
signals turned ON and OFF in synchronization with the supply
voltage. Also it is a characteristic that the voltage across the
load can be heightened and lowered by the phase-lead of the current
in an inductive load.
[0008] When too much magnetic energy caused by an overcurrent of a
load is regulated by a normal operation of these reverse-conductive
type semiconductor switches, it may result in that an unexpectedly
large level of magnetic energy is recovered, and the level may
exceed the withstand voltage of the capacitor and/or the withstand
capacity for the current and voltage of the reverse-conductive type
semiconductor switches. In this instance, if the reverse-conductive
type semiconductor switches can be protected immediately, such
protection will also protect the load and the power supply. This
suggests the significance of the switch protection functionality,
and at the same time implies its another importance for achieving
the downsized, low-cost switch, which will be attained when the
switch itself is free of a overload capacity requirement as a
result of the above protection capability.
[0009] FIG. 1 shows an AC power supply unit using magnetic energy
recovery switch of which a patent application has been filed and
has also been publicized (see Unexamined Japanese Patent
Application KOKAI Publication No. 2004-260991). In this structure,
when a resistance componentof the load R decreases for a short
period of time transiently, a large current flows and magnetic
energy increases, and the capacitor C cannot absorb any more
magnetic energy and overvoltage takes place. At this time the
reverse-conductive type semiconductor switches (S1-S4) are applied
with the same voltage and may be destroyed when the voltage applied
exceeds the withstand voltage thereof.
[0010] FIG. 2 shows, by computer simulation, that when a resistance
component of the load R decreases to a half and an overcurrent
flows and, as a result, voltage of the capacitor rapidly increases.
The peak of the capacitor voltage rises from 200V to 700V 0.5
seconds after the accident took place. This shows that the merit of
affordability of a smaller-sized capacitor thanks to the confined
requirement to store solely the magnetic energy of the load, can
serve as a demerit of less energy absorption capacity in case of an
overcurrent.
[0011] In order to protect this kind of reverse-conductive type
semiconductor switches, it is easy to stop the operation thereof by
moving to a bypass mode (short-circuit). This will end up in
stopping the operation of all the loads and leads to stop, at the
same time, operation of the other machinery which are connected and
in the midst of operation. This was considered unavoidable before.
This magnetic energy recovery switch is provided with an energy
storage capacitor which recovers magnetic energy with four
reverse-conductive type semiconductor switches, and can set freely
timings for breaking and making of the circuit. If an operation
control method which will utilize such merit fully enough is taken,
it is possible to achieve stopping and/or limiting the current with
the maximum capacity without merely breaking the circuit at the
time of overcurrent, and returning to normal condition when the
cause of the overcurrent is removed. This means that the magnetic
energy recovery switch can serve as a highly-functional AC
switch.
[0012] The present invention is realized in consideration of the
above described conditions, and it is an object of the present
invention to provide a magnetic energy recovery switch having a
protective circuit for protecting the magnetic energy recovery
switch against overvoltage and/or overcurrent caused due to an
abnormality and/or a trouble of the load.
Means for Solving the Problem
[0013] The present invention relates to a magnetic energy recovery
switch having a protective circuit and interposed between an AC
power supply and a load to protect, against overvoltage and
overcurrent, the magnetic energy recovery switch having a variable
reactance function for turning the ON/OFF of the AC current and
changing a phase thereof. The above object of the present invention
will be achieved by a magnetic energy recovery switch having a
protective circuit, the magnetic energy recovery switch
comprising:
[0014] a bridge circuit constructed from four reverse-conductive
type semiconductor switches;
[0015] a capacitor connected between DC terminals of the bridge
circuit, recovering and storing magnetic energy at cut off of a
current; and
[0016] control means;
[0017] wherein the protective circuit comprises:
[0018] a voltage detection unit connected in parallel with the
capacitor to detect a voltage of the capacitor; and
[0019] a discharge circuit connected in parallel with the capacitor
and having a discharge resistor and a discharge switch connected in
series; and
[0020] wherein the control means controls a phase of a gate control
signal so as to simultaneously turn ON/OFF opposed pairs of the
reverse-conductive type semiconductor switches of the bridge in
synchronization with the voltage of the AC power supply, and also
and controls the gate of the discharge switch so as to
short-circuit the discharge switch to discharge electric charge of
the capacitor through the discharge resistor when an output of the
voltage detection unit exceeds a predetermined value.
[0021] Also the above object of the present invention can be
achieved by a magnetic energy recovery switch having a protective
circuit, wherein the protective circuit further comprises a current
detection unit interposed between the AC power supply and the load
and detecting the current which flows to the load; and the control
means controls to limit a current by making the duty ratio of
ON/OFF of the pulse of the control signals smaller than 0.5 when
the output of the current detection unit exceeds a predetermined
value.
[0022] Moreover the above object of the present invention can be
effectively achieved by making the control means control the gates
so as to turn OFF all the four reverse-conductive type
semiconductor switches to cut off the current when a length of time
during which the output of the voltage detection unit is in excess
of a predetermined value exceeds a predetermined time period; or by
making the control means control the gates so as to turn ON , when
the capacitor voltage is zero, all the four reverse-conductive type
semiconductor switches, to achieve a bilateral conducting state,
when the length of time during which the output of the voltage
detection unit is in excess of a predetermined value exceeds a
predetermined time period.
[0023] Furthermore, the above object of the present invention can
be effectively achieved by making the protective circuit further
comprises a current detection unit interposed between the AC power
supply and the load to detect the current which flows to the load,
wherein the control means controls the gates so as to turn OFF all
the four reverse-conductive type semiconductor switches to cut off
the current, when the output of the current detection unit exceeds
a predetermined value; or the control means controls the gates so
as to turn OFF only one reverse-conductive type semiconductor
switch of the opposed pair of the reverse-conductive type
semiconductor switches of the bridge which are ON to cut off the
current when the output of the current detection unit exceeds a
predetermined value.
BRIEF DESCRIPTION OF DRAWINGS
[0024] FIG. 1 shows an example of an AC power supply unit using a
conventional magnetic energy recovery switch;
[0025] FIG. 2 shows results of simulation of the overvoltage and
overcurrent generation;
[0026] FIG. 3 is a circuit block diagram showing a structure of a
magnetic energy recovery switch having a protective circuit
according to the present invention;
[0027] FIG. 4 shows a disruption in the current wave form due to a
step change in a gate control phase;
[0028] FIG. 5 shows a current control by controlling the duty ratio
of a gate pulse;
[0029] FIG. 6 shows a current control by protection against an
overvoltage;
[0030] FIG. 7 shows a simulation model and results thereof;
[0031] FIG. 8 is a circuit block diagram for a simulation;
[0032] FIG. 9 shows analytical results obtained by a
simulation;
[0033] FIG. 10 shows effects of turning OFF all the gates by
simulation; and
[0034] FIG. 11 shows simulation results about a control through
stopping the gates of S1 and S4 when an instantaneous value of the
current exceeds a predetermined value.
BEST MODE FOR CARRYING OUT THE INVENTION
[0035] This invention relates to a magnetic energy recovery switch
having a protective circuit to protect magnetic energy recovery
switch against overvoltage and/or overcurrent. FIG. 3 shows a
preferable embodiment of the present invention.
[0036] The magnetic energy recovery switch (hereinafter, MERS)
provided with a protective circuit of the present invention
comprises a bridge circuit constructed from four reverse-conductive
type semiconductor switches (hereinafter, semiconductor switches)
S1-S4, a capacitor 2 connected between DC output terminals of the
bridge circuit for recovering and storing the magnetic energy
(snubber energy) of the circuit when the current is cut off, and
control means 4 for controlling the phase of the gate control
signals so as to turn simultaneously ON/OFF opposed pairs (S1 and
S3, S2 and S4) of the semiconductor switches of the bridge. This
MERS is interposed in series between an AC power supply 3 and loads
(L, R).
[0037] The protective circuit for protecting MERS against
overvoltage and/or overcurrent comprises a voltage detection unit 5
connected in parallel with the capacitor 2 for detecting the
voltage of the capacitor 2 and a discharge circuit 6 connected in
parallel with the capacitor 2 and having a discharge resistor 61
and a discharge switch 62; and the ON/OFF of the discharge switch
62 is controlled by the gate control signals supplied from the
control means 4. More concrete explanation is that an output of the
voltage detection unit 5 is inputted into the control means 4, and
the output value is compared with a threshold value previously
stored at the control means 4. When the output of the voltage
detection unit 5 exceeds the threshold value, that is, when the
capacitor has an overvoltage, the control means 4 sends ON signal
to the gate of the discharge switch 62 to short-circuit the
discharge switch 62 for discharging electric charge of the
capacitor 2 through the discharge resistor 61 to reduce the
capacitor voltage; and the control means 4 sends OFF signal to the
gate of the discharge switch 62 and the discharge switch gets
turned OFF, when the capacitor voltage returns to a value within a
normal value area. Power MOSFETs and IGBTs or the like can be used
for semiconductor switches.
[0038] In case that a discharge circuit 6 for protecting the
capacitor 2 by discharging the capacitor 2 is added, when the
capacitor voltage exceeds 400V as shown in FIG. 4, the discharge
switch 52 is turned ON to operate the discharge circuit 6 to
discharge the current to the discharge resistor 61, thereby
preventing the capacitor voltage from exceeding 400V. What is
important is that as a result of the reduced capacitor voltage, the
reactance voltage is also reduced and, therefore, the rising of the
load current is also reduced due to the limitation of the capacitor
voltage. It is a characteristic of the overcurrent of the magnetic
energy recovery switch that the capacitor voltage rises rapidly,
and if the capacitor voltage is reduced, the current does not rise.
This is essentially different from the protection of a capacitor
against overvoltage by the conventional inverter device.
[0039] As another means it may be possible that when a magnetic
energy recovery switch or a capacitor exceeds a withstand voltage,
the phase of the gate control signals of the magnetic energy
recovery switch is further advanced to reduce shared voltage of the
load. However, controlling cycle of the phase of the gate pulse is
a half cycle in synchronization with the power supply, therefore,
the change of the ON/OFF phase which is close to the phase speed
makes the ON time of the gate longer than a half cycle and
generates a DC component at a load, which is not desirable. For a
change in ON/OFF phase it is necessary to put a time consonant of
more than 10 mS. The result will appear one cycle after. As for
controlling the change in the output of the load, the normal
control as described above is enough. However in the case of an
overcurrent due to an accident or the like, the speed of increasing
current is faster than one cycle and the response is not in time.
FIG. 5 shows simulation results illustrating that because of a
rapid change in the phase of the gate signals, pulses are lacking
and there is a disruption in the output current wave form.
[0040] In another preferable embodiment of the protective circuit
of the present invention, a method to reduce the pulse width of the
ON signal of the gate (that is, to make the duty ratio of the gate
pulse signal smaller than 0.5) is taken.
[0041] That is, detection of a current flown to the load is carried
out by the current detection unit 7 interposed between the AC power
supply 3 and the load, and when the output of the current detection
unit 7 exceeds a predetermined value, the control means 4 controls
to make the ON/OFF duty ratio of the pulse of the gate control
signals of the semiconductor switch 1 smaller than 0.5, thereby
carrying out a current limiting control.
[0042] By combining the discharging of the capacitor by the
discharge circuit 6 with the current limiting control through
making the ON/OFF duty ratio of the pulse of the gate control
signals of the semiconductor switches 1 smaller than 0.5,
protection function of the MERS is further improved.
[0043] FIG. 6 shows results of a simulation in which the pulse
width of ON signal is rapidly reduced from 180.degree. after 0.65
seconds and made return to the normal width after 0.85 seconds.
Although the pulse width was reduced rapidly, the current wave form
was not disrupted and the current was decreased.
[0044] After all, the combination of the protection function
against overvoltage by a voltage peak cut for the capacitor and
current limiting function by the pulse width control of the ON
signal of the gates can be widely applicable as protection method
for MERS, and yet protection is possible only through such a
combination in this instance.
[0045] The present invention is provided with, against an
instantaneous overvoltage, a discharge circuit 6 for discharging
electric charge of the capacitor which is overcharged, and, first
of all, protection is carried out for the overvoltage of the
capacitor 2 and the semiconductor switch 1 by a most simple and
effective means. Unlike a voltage-type converter, the short-circuit
of MERS provides no problem as the AC power supply 3 and the load
are just connected in series. Discharge resistor 61 may be
determined by the current capacity of the discharge circuit 6 and
also by the heat capacity of the discharge switch 62; however, 10
times or 20 times the on resistance of the discharge switch 62 will
be one standard. In the embodiment 10.OMEGA. is used.
[0046] It is possible to use a thyristor for this discharge switch
62 which, however, will stop the operation as MERS and the leading
reactance voltage will become zero and, therefore, the reactance on
the whole becomes large. This is a function as a current limiter,
and is an important function. However, as the energy of the
capacitor is completely discharged, the capacity of the discharge
resistor 61 must be enlarged. Also voltage non-linear element such
as ZNR or CYDAC may be used instead of the discharge switch 62.
[0047] The control means 4 of the MERS decreases the pulse width of
the ON signals of the gates either by an overvoltage signal when an
overvoltage is detected or by an overcurrent signal when an
overcurrent is detected, or by the combination of both signals. By
including to allow the signal to be instantaneously changed to OFF
from ON, the rising of voltage stops and load current is lowered
under the overcurrent protection level by current feed back
control. The overvoltage of the capacitor is also lowered under the
protection level. It is new that the discharge by the discharge
switch 62 and the current feed back control by controlling the duty
ratio of the gate pulse are combined, thereby making the MERS
operate as a current limiter through the current limiting control
is possible without semiconductor switches thereof getting
destroyed. As a result, when the start-up rush of the load is over,
MERS can return to the normal operation automatically, which marks
an important starting point for MERS to possess a more intelligent
function in addition to turning ON/OFF the switch. The current
limiting operation at the time of an accident has been being
researched centering on a variety of current limiters and such
research has not been accomplished yet. The MERS of the present
invention is an ideal AC current switch and, therefore, the present
invention can provide a semiconductor-type static current limiter
which can limit the current by reducing the capacitor voltage even
when gates are all turned OFF. Such operation became possible owing
to the advancement of the semiconductor, and when IGBT especially
developed for MERS with conduction loss of 1.54V (the same with
thyristor) is used as AC semiconductor switch, such an intelligent
AC semiconductor switch can be provided as normal conduction loss
thereof is small, and it is not necessary to stop operation for the
protection at the time of accident or for the only initial start-up
rush current, and the operation can be continued and wait for the
transient condition to pass limiting the current within the
overload capacity predetermined for the circuit.
[0048] FIG. 3 shows an embodiment of the present invention, in
which semiconductor switches 1 are arranged in a bridge structure
and a capacitor 2 for storing the magnetic energy is connected
between the DC terminals. Unlike the capacitor of the conventional
voltage type inverter, this capacitor is only for storing the
magnetic energy of the load and, therefore, the capacity of the
capacitor can be small. It is a characteristic of this capacitor
that at an each half of the cycle the voltage reaches a peak and
becomes zero voltage through a discharge. The wave form of the
current for charging and discharging should be approximated to the
angular velocity .omega..sub.0. As a result, higher harmonics
decrease. The relation between the capacitance C and the AC
inductance L is as follows:
LC=.omega..sub.0.sup.-2 (equation 1)
[0049] Moreover, by making the value of the capacitance C slightly
smaller than the value acquired from the above equation, there is a
period generated when there is no voltage after the discharge of a
half cycle, thereby switching of semiconductor switches 1 becomes
easier. It is a characteristic that the voltage supply capacitor of
a single-phase inverter has, unlike the conventional PWM converter,
a drastically small capacitance.
[0050] As the voltage of the capacitor 2 oscillates with the gate
cycle, protection against an overvoltage must be carried out in a
high speed. If the capacitor voltage detected by the voltage
detection circuit 5 is likely to exceed the threshold value, the
voltage is discharged by the discharge resistor 61 for the current
limiting or the like; as a result, the voltage of the capacitor
remains at the value without exceeding the threshold value.
[0051] Also receiving separately the detection signal from the
current detection circuit 7, the pulse width of the ON signals of
the gates (duty ratio) is made smaller than 0.5 in order to keep
the threshold value of the overcurrent.
[0052] An explanation will be given using the simulation results in
FIG. 7. When the detected current exceeds the threshold value, the
control means makes the duty ratio of the gate control pulse
smaller than 0.5, thereby reducing the ON time of the gates. This
does not bring about effects thereof instantaneously, and the
discharge circuit 6 for the capacitor is also necessary. When the
current exceeds the threshold value, the OFF time of the gates of
the semiconductor switches 1 becomes longer (the pulse width of ON
becomes narrow). As a result, the semiconductor switches 1 serve as
a current limiter. The maximum inverse voltage generated is a
voltage of the protection level for the capacitor.
[0053] As the capacitor's withstand voltage is larger than 1.4
times the supply voltage, the current reduces.
[0054] The control means 4 has a capacity to detect the voltage
phase of the AC power supply 3 and sends out gate signals necessary
for the four semiconductor switches 1. Although the control means
sends gate signals simultaneously to the pair of S1 and S3 and to
the pair of S2 and S4 of opposing semiconductor switches, the
control means does not send ON signals to the two pairs
simultaneously. It is because the capacitor voltage gets
short-circuited.
[0055] The relation between the AC supply voltage and the gate
pulse signals is shown in the top trace in FIG. 7. Here the width
of the gate pulse signal is kept at the same cycle of the basic
wave, and the duty ratio is controlled so that only the ON time is
shortened.
[0056] In this instance, the overcurrent makes the magnetic
(snubber) energy unexpectedly large, and the capacitor has an
overvoltage once; such period, however, is a short time period
before the current limiting function brings about an effect after a
few scores of micro seconds. This way it is important to send a
limited electric power without stopping the switches even at the
time of an accident by means of a protective circuit combining
instantaneous voltage limiting function (discharge) and overcurrent
inhibition function for removing the reasons for the
overvoltage.
Simulation of an Embodiment
[0057] FIG. 8 shows a circuit block diagram for a simulation and
FIG. 9 shows analyzed results thereof. The circuit consonants are
as follows:
[0058] 1. semiconductor switches: S1, S2, S3, S4 (Power MOSFETs;
conducting loss is ignored.)
[0059] 2. AC power supply: 50 Hz, AC 100V
[0060] 3. load inductance L: 31.85 mH
[0061] 4. load resistance R: 10.OMEGA.
[0062] 5. load resistance at abnormal time R': 2.OMEGA.
[0063] 6. duration: 0.1 s
[0064] 7. capacitor: 150 .mu.F, overvoltage protection level
(threshold value) 400V
[0065] A case in which a load resistance value is drastically
changed after 0.5 seconds thereby causing a rapid increase in the
current is simulated. As a result of the increase in the current,
the capacitor now has an overvoltage and the discharge circuit gets
in motion and the voltage is cut at 400V. When the overcurrent is
detected by the current detection unit 7, the pulse width of the
gate ON signals is reduced and after 0.1 seconds the current
reduced, and, as a result, the overvoltage was dissolved.
[0066] Also watching an instantaneous value of the current is
carried out and when the current exceeds a predetermined value, all
the gates are turned OFF, and the rising of the current can be
turned to decreasing. Effects of the turning OFF of all the gates
are shown in simulation in FIG. 10. This has a great effect when
the load is resistive. In the circuit of FIG. 8, when the resistive
component of the load (L=10 mH, R=10.OMEGA., R'=10.OMEGA.) has,
after 0.06 seconds, become a half from 10.OMEGA. to 5.OMEGA., the
switch current rises. A watch level is set to 20 A, and the
simulation results of a case in which all the gates of S1, S3 and
S2, S4 are turned OFF, when the current is detected to be 20 A, are
shown. As a result, the rising of the current stops at 20 A,
thereby protecting the switch.
[0067] Moreover, when the gate signal for one of the opposed pair
of the switches is stopped, the capacitor current is stopped and
the capacitor discharge is stopped and voltage decrease stops.
Especially when the load is inductive, the rising of the current
also stops and, as a result, something like a current limitation
becomes possible. This can be utilized, when the load is inductive,
as a generating device of leading current (SVC: Static VAR
Compensator) for controlling the leading current. Computer
simulation results of controlling by turning off the gates of S1
and S4, when the instantaneous value of the current exceeds a
predetermined value, are shown in FIG. 11. The circuit used for
this simulation has a similar circuit as in FIG. 8 and when an
inductive load (L=30 mH, R=10.OMEGA., R'=0.5.OMEGA.) is connected,
the current limiting is possible by turning OFF one gate of the
pair at 15 A, for example. This is a control for a case of the load
with a low power factor or for a case of acquiring leading current
by flowing the current through a coil. The current control is
carried out by turning OFF the gates of S1 and S4 at 15 A.
[0068] The discharge resistor 61 has to withstand the rising
temperature when the input energy becomes excessively large. In
this embodiment, however, discharge current is flown only during
such time period as the capacitor voltage exceeds a predetermined
voltage. When the discharge resistor 61 becomes overloaded, two
methods remain as the final protection.
[0069] One of them is to stop all the current by turning OFF the
gates of all the switches of the MERS as described above, and the
other is to short-circuit the MERS. In this instance the capacitor
voltage decreases to zero or almost zero at the frequency twice the
gate frequency. Aiming at this timing when the voltage is close to
zero, gate signals now turn ON/OFF simultaneously different pairs
of S1, S2 and S3, S4 while the gate signals used to be given
simultaneously to the opposed pairs of the semiconductor switches
until then. This way the AC current now comes to be in a bypass
condition and yet the capacitor charge can remain without being
short-circuited when the change-over is carried out.
[0070] This embodiment was explained with a single phase circuit;
however, by using three sets of MERS, an embodiment naturally can
be applied to a three-phase AC. In this instance there is such an
effect as the harmonics of a tertiary current by the Star-Delta
Transformation disappear. Furthermore, response at the time of an
unbalance accident of the three-phase is possible.
[0071] The control means of this protective circuit has a
capability to judge rationality of the control signals from
outside. First of all is that there is an identification number for
an individual switch and this can be a key for the communication
with outside. This function, for example, can make it possible that
when radio signals are sent out to the switch via communication
system such as Internet, the switch, then, becomes to be able to be
controlled by radio control; and, thus, control signals can be
conveyed without wire connection.
[0072] The control means of this protective circuit acquires not
only the current, voltage, phase, power factor but also has a
function to find out an abnormality through checking the soundness
from impedance acquired from voltage and current. Moreover it is
possible to control semiconductor switches accordingly in response
to the operational condition of the load acquired from the
computation.
[0073] The control means of this protective circuit stores the
operational conditions of these semiconductor switches in the past
and it may be possible to send out information to outside in
response to a request therefrom by integrating the total operation
time, the electric power, the consumed electric power or the
like.
[0074] This protective circuit has the common function for all the
MERS even with loads having different capacities and objects. This
protective circuit will be required and by producing the circuit
separately from the main circuit, it is possible to realize a cost
down by the effect of a mass production; so it is better to
standardize the circuit and also to make structure thereof easy to
be attached.
[0075] As the control means of this protection circuit has a
programmed computing function, it is possible to make the content
downloaded and uploaded via communication function from outside. It
is also possible to reflect the characteristics and operation plan
of the load in the program.
[0076] The control means of this protective circuit has a control
computing function and a storage function, and, therefore, in the
case of application thereof to a lighting fixture, it is possible
to control the fixture through detecting the luminance of the floor
surface or making the luminous efficiency a function in a case of a
fluorescent lamp or the like when the luminous efficiency changes
according to the outside temperature.
* * * * *